1. Field of the Invention
The present invention is related to centrifugal blood pumps, and in particular to the use of centrifugal blood pumps during open-heart surgery.
2. Description of the Prior Art
Centrifugal pumps have been used for many years to pump a wide variety of different fluid materials. In general, a centrifugal pump includes a pumping chamber with an inlet aligned with a rotational axis of the pump, an outlet adjacent the periphery of the pumping chamber, and an impeller mounted within the pumping chamber for rotation about the axis. The impeller in such pumps can be mounted on a drive shaft which extends outside the pumping chamber to a rotational drive source or the shaft can be mounted within the pumping chamber as a spindle about which the impeller rotates (rotatably driven by means other than the rotation of the shaft, such as a magnetic drive arrangement). In any case, as the impeller is rotated, it imparts centrifugal force and velocity to the fluid, thus pumping the fluid from the pump inlet to the pump outlet.
In recent years, centrifugal pumps have been used extensively for pumping blood during open heart surgery. Examples of centrifugal blood pumps are shown in the following U.S. patents: Rafferty et al U.S. Pat. No. Re. 28,742; Dorman et al U.S. Pat. No. 3,608,088; Rafferty et al U.S. Pat. No. 3,647,324; Kletschka et al U.S. Pat. No. 3,864,055; Rafferty et al U.S. Pat. No. 3,957,389; Rafferty et al U.S. Pat. No. 3,970,408; Rafferty et al U.S. Pat. No. 4,037,984; and Reich et al U.S. Pat. No. 4,135,253.
In those heart/lung bypass systems using a centrifugal pump, an arterial flow line is connected between the outlet of the centrifugal pump and an artery of the patient, and a venous flow line is connected between a vein of the patient and the inlet of the centrifugal pump. An oxygenator system is also provided at some point in the bypass system. When a bubble oxygenator is used, it is typically placed in the venous flow line between the patient and the pump. When a membrane oxygenator is used, a venous reservoir is usually placed in the venous flow line between patient and pump and the membrane oxygenator itself is placed in the arterial flow line between pump and patient.
Proper operation of the heart/lung bypass system requires that the outlet pressure of the pump exceed the blood pressure from the patient, and other pressure factors (such as the relative height of the patient and the system components) which combine to define a "load" pressure, so that flow of blood is from the outlet of the centrigual pump to the patient and from the patient back to the inlet of the pump. If the load pressure exceeds the outlet pressure of the pump, flow will be in the reverse direction, and the system will not be performing its function of replacing the heart and lungs during the open-heart surgery.
The sensing of blood flow in the heart/lung bypass system is preferably performed by a noncontacting type of flowmeter. This reduces the chance of contamination or damage to the blood by contact. In addition, since all parts of the system which contact the blood must be disposed of or resterilized after a single use, a noncontacting type flowmeter is preferable since it can be reused without resterilization.
One particularly advantageous type of noncontacting flowmeter for sensing blood flow is an ultrasonic Doppler flowmeter. While there are significant advantages (including noncontacting operation and high accuracy) provided by ultrasonic Doppler flowmeters, these types of flowmeters provide an indication only of flow magnitude, but not flow direction. An ultrasonic flowmeter, therefore, cannot distinguish between flow from the centrifugal pump to the patient and reverse flow from the patient to the centrifugal pump.